The present invention relates to a numerically controlled machine tool such as a roll grinder or the like for grinding a workpiece into a roll, and more particularly to a method of correcting the misalignment of a workpiece on a numerically controlled machine tool, which is caused when the axis of the workpiece is displaced off the axis of the machine tool, by automatically correcting numerical control data according to the misalignment.
Numerically controlled machine tools such as roll grinders or the like for grinding a workpiece into a roll are generay arranged to operate by rotating the workpiece about its own axis at a given speed, and moving a tool back and forth radially of the workpiece while moving the tool axially of the workpiece. Since the machine tools are required to move the tool in the above manner, the machine tools have memory mean for storing numerical control data based on which amounts of radial movement of the tool at respective axial positions thereof are successively programmed, and a control unit for distributing command pulses to an actuator for moving the tool based on the stored numerical control data. The machine tools are widely used to machine rolls for use in rolling mills, or he like.
A workpiece to be machined by such numerically controlled machine tool has to be accurately attached along the axis of the machine tool. However, inasmuch as a workpiece to be machined into a roll for use in a rolling mill is large in volume and weight, it is quite difficult to mount the central axis or axis of rotation of workpiece in exact alignment with the axis of the machine tool. If the workpiece as mounted on the machine tool is tilted with respect to the axis of the machine tool, the workpiece will be misaligned in a machining process.
The numerical control data stored in the memory means of the numerically controlled machine tool are established with reference to coordinates in the axial direction of the machine too. Therefore, if a misaligned workpiece is machined into a roll, the circumferential surface of the completed roll is not ground parallel to the central axis thereof, but ground to a tapered shape. In order to prevent a workpiece from being misaligned on a numerically controlled machine tool, it is necessary to install the workpiece in an accurate centering process, which is, however, poor in efficiency.
In practice, centering a workpiece several times during a machining process has been done in order to eliminate irregularities in supporting positions on the opposite ends of the workpiece which may have been caused by low centering accuracy and temperature fluctuations. This procedure is carried out by mechanisms positioned respectively at the headstock and the tailstock of the machine tool for sliding the workpiece with respect to the axis of the machine tool. These mechanisms include motors for sliding the headstock and the tailstock, respectively, to effect centering adjustments. This arrangement, however is a complex machine tool construction, and results in an increased machining time and poor efficiency since centering adjustments are required during a machining process.